The worldwide production of chlorine is about 45 millions of tons per year, about 20 of which are produced by electrolysis of a sodium chloride solution by the diaphragm electrolysis process.
FIG. 1 schematizes a modern diaphragm cell comprising an anode base (A) having the anodes (B) fixed thereto by means of a copper conductor bar (D) protected by a titanium layer. The cathode (C) is made of perforated plates or iron meshes on which a diaphragm is deposited from the anode side. The cover (G), made of a plastic material resistant to chlorine, is provided with an outlet (H) for the chlorine gas and an inlet for the feed brine (not shown in the figure).
Hydrogen and caustic soda are withdrawn from the cathode compartment through outlets (I) and (L) respectively. The diaphragm, substantially made of asbestos fibers and a plastic binder, separates the anode compartment from the cathode compartment avoiding mixing of the two gases and of the solutions (catholyte and anolyte)
In view of its technical-commercial importance, the diaphragm process technology has been recently improved to reduce the energy consumption and to avoid the use of asbestos which is considered a dangerous agent for the human health, by resorting to diaphragms made of zirconium oxide fibers and plastic materials, such as polytetrafluoroethylene.
Among the various other developments introduced in the diaphragm process, particularly important under an industrial point of view are:
1. replacement of graphite anodes with DSA.RTM. anodes of the box type; PA1 2. replacement of box-type anodes with expandable anodes (U.S. Pat. No. 3,674,676); PA1 it is extremely difficult inserting and welding two superimposed expanders and even more a new expander onto an existing one, which is usually deformed after prolonged use. As a consequence the two expanders do not match and in the contracted position there is a "surplus of material" which deformates the expander. PA1 Deformation of the expanders in the contracted position poses problems for both the insertion of the retainers and for positioning the anodes onto the anode base to obtain a good planarity The problems affecting the expanders in the contracted position seriously influence the expansion action when the retainers are removed. Consequently the pressure on the diaphragm is not uniform, the two active surface are not sufficiently parallel and the distance of the same from the diaphragm is not constant. The operation of the anode and of the diaphragm is therefore badly affected. PA1 lower voltage drop in the expander without negatively affecting its flexibility PA1 parallel expansion of the anode surfaces PA1 uniform pressure onto the diaphragm, remaining constant with time PA1 lower voltage drop at the welding points between the conductor bar and the expander.
3. anode in contact with the diaphragm in the so-called "zero gap" configuration (U.S. Pat. No. 5,534,122). This result is obtained introducing inside the expandable anode suitable devices capable of applying pressure against the anode surfaces.
FIG. 2 shows a typical expandable anode comprising two anodic surfaces (E), connected to the conductor bars (D) by means of flexible sheets called expanders (F), which, during assembling, are kept in the contracted position by the so-called retainers (N). The retainers are removed after assembly to let the anodic surfaces (E) expand. It is clear that the expanders have not only the function to make the two anodic surfaces (E) mobile but also to make electric current flow from the vertical conductor bar (D) to the anodic surfaces (E). In order to ensure a sufficient elasticity the expanders are made of a thin titanium sheet, for example 0.5 mm thick. As a consequence, a remarkable voltage drop localized in the expanders is experienced, about 1-2 times higher than that typical of the box anode. For example, a conventional box anode of the MDC 55-type cell, operating at 2.5 kA/m.sup.2, 95.degree. C., has a voltage drop of 40-50 mV vs. 100-120 mV of a similar expandable anode. Likewise, a conventional box anode of the MDC 29-type cell, operating at 2.5 kA/m.sup.2, 95.degree. C., has a voltage drop of 50-60 mV vs. 110-130 mV of a similar expandable anode.
The invention described in Brazilian Patent Application No. Pl9301694 suggests a solution to reduce the ohmic drop in the expander. The invention consists in welding two or more superimposed expanders having the same thickness (0.5 mm) in order to increase the cross section for the electric current flow and avoid a reduction of elasticity. In practical applications this solution proved to be far from optimum and did not find any industrial applications up to now, due to the following reasons:
It must be noted that welding of one or more superimposed expanders causes a remarkable voltage drop at the interface between the copper core of the conductor bar and the titanium layer. Discontinuities are created at this interface as a consequence of the increased thermal stress during the welding procedure (higher temperature for a longer time). These effects are decidedly negative when welding of the second expander is made on existing conductors already deteriorated after years of operation. The voltage saving is even completely nullified when three superimposed expanders are welded.